Subcritical Flaw Growth in 9Ni-4Co-0,25C Steel - A Fatigue and Fractographic Investigation and Its Relationship to Plane Strain Fracture Toughness
Abstract
Fatigue crack propagation and plane strain fracture toughness (K(sub Ic)) studies were conducted on a 9Ni-4Co-0.25C steel which had been quenched and tempered to a yield strength of 180 ksi. Fatigue specimens were cycled to failure in a room air environment under several combinations of cyclic and tensile mean loads. It was observed that fatigue crack propagation became unstable at a critical level of stress-intensity which led to rapid failure. This critical stress-intensity level in fatigue was consistently below the stress-intensity value for crack instability predicted by conventional rising load K(sub Ic) tests. Electron fractography studies were conducted on the fatigue surfaces. A gradual change in fracture surface appearance with cyclic stress-intensity level was found to occur. Fatigue striations predominated at low cyclic stress-intensity levels, changing to dimpled rupture at high cyclic stress-intensity levels. No abrupt change in fracture surface appearance was observed to coincide with the critical stress-intensity level in fatigue. It is concluded from this study that, although K(sub Ic) provides a lower-bound estimate of stress-intensity for terminal fracture in monotonic loading, subcritical flaw growth by fatigue in some high-strength materials may define a more conservative estimate of structural material performance.
Document Details
- Document Type
- Technical Report
- Publication Date
- May 01, 1968
- Accession Number
- ADA405714
Entities
People
- C. N. Freed
- E. A. Lange
- L. A. Cooley
- Thomas W. Crooker
Organizations
- United States Naval Research Laboratory